GI 1 Flashcards
What is the function of the colon?
- Reabsorbs fluids and electrolytes
- Stores faecal matter before delivery to the rectum
What are the four layers of the digestive tract wall?
1) Mucosa
2) Submucosa
3) Muscularis externa
4) Serosa
What produces motility in the GI tract?
- Gap junctions produce synchronous waves between coupled cells
- Spontaneous electrical activity from Interstitial Cells of Cajal (ICCs)
- Modulated by intrinsic and extrinsic NS and hormones
How do ICCs work?
- Form bridge between plexus nerve endings and smooth muscle cells
- Contraction occurs when slow wave amplitude reaches threshold
- Force related to duration that slow wave is above threshold
How does peristalsis occur?
- Distension of gut activates sensory neurones
- Altered activity of interneurones and motoneurones
- Longitudinal/circular muscle relax + contract
What are the major motility patterns in the GI tract?
1) Peristalsis
2) Segmentation
3) Colonic mass movement
4) Migrating motor complex (MMC)
5) Tonic/sustained contractions
What is gluconeogenesis?
The de novo synthesis of glucose from metabolic precursors (lactate, amino acids, glycerol); primary source of glucose overnight, but energetically expensive
Describe the process of glycogen breakdown
1) Glycogen -> G1P by glycogen phosphorylase (RLS)
2) G1P -> G6P by phosphoglucomutase
3) G6P -> glucose in liver by glucose-6-phosphatase or to glycolysis
Why is fat required in the diet?
- Fat-soluble vitamins
- Essential fatty acids
- As an energy source
What are the main products of fat digestion?
1) Glycerol; readily absorbed into epithelial cells
2) FAs; short&med enter portal blood
3) Monoglycerides; with long FAs -> triglycerides
What are chylomicrons?
Absorbed fats coated with a layer of protein, phospholipid or cholesterol; enter lymph -> bloodstream -> muscle and adipose tissue -> attacked and cleaved by lipoprotein lipases -> provide energy/storage
What epithelium is found in the oral cavity, oropharynx and laryngopharynx?
Non-keratinised stratified squamous epithelium
What epithelium is found in the nasal cavity and nasopharynx?
Respiratory epithelium // ciliated columnar epithelium
Describe the epithelium on the anterior 2/3rds of the tongue
Stratified squamous epithelium; thin on ventral surface and thick with papillae on dorsal surface
Describe the epithelium on the posterior 1/3 of the tongue
Smooth stratified squamous epithelium which, except for the circumvallate papillae, lacks papillae but does have substantial lymphoid aggregates in the submucosa
What occurs at the gastro-oesophageal junction?
The abrupt transition from stratified squamous epithelium of oesophagus to the simple columnar epithelium of the cardia of the stomach
Describe the distribution of cells in the gastric pit
1) Isthmus (stem) - parietal cells and stem cells
2) Neck (junction) - mucous cells and parietal cells
3) Fundus (base) - mostly chief cells; also parietal and neuroendocrine cells
What is the difference between chief and parietal cells of the gastric glands?
- Chief cells are digestive enzyme secreting cells
- Parietal cells are hydrochloric acid producing cells
What is different about the muscular externa of the stomach?
It contains an additional layer which is oblique and internal to the circular muscle layer; it aids the churning action of the stomach
What occurs at the gasproduodenal junction?
An abrupt transition from stomach mucosa to duodenal mucosa; the inner circular layer of smooth muscle to thickened to form the pyloric sphincter
Describe the duodenum of the small intestine
25cm long; contains Brunner’s glands in the submucosa
Describe the jejunum of the small intestine
2m long; tallest villi, located on permanent circular folds of the mucosa and submucosa, the plicae circularis; lymphoid follicles infrequent
Describe the ileum of the small intestine
2.75m long; characterized by shorter villi and aggregations of lymphoid follicles called Peyer’s patches found in the submucosa and often extending into the lamina propria
What occurs at the rectoanal junction?
Distinct junction between the mucosa of the rectum and the non-keratinized stratified squamous epithelium of the anal canal which is continuous with the keratinized stratified squamous epithelium of the surrounding skin
What is the function of the liver?
- Synthesis and secretion of bile
- Synthesis of special proteins
- Detoxification
- Storage of glycogen, vitamins and iron
Describe the structure of the liver
Made up of hexagonal segments called lobules with a branch of the portal vein in the centre and a portal triad at each corner; hepatocytes form sheets with spaces for blood channels called sinusoids
What is the space of Disse/perisinusoidal space?
A narrow space between the lining endothelial cells of sinusoids and the hepatocytes
What are hepatic stellate cells (Ito cells)?
Found scattered in the space of Disse; modified fibroblasts + make CT; store vitamin A within fat droplets in their cytoplasm; can transform into myofibroblasts to make scar tissue
What are Kupffer cells?
Macrophages found within sinusoids which remove worn out red blood cells and particulate matter from the blood
How is bile produced?
Hepatocytes produce bile which flows via bile cannaliculi to bile ducts and then to hepatic ducts
What is the function of the gall bladder?
To store and modify bile received from the liver
Describe the structure of the gall bladder
A muscular sac lined by simple columnar epithelium, a lamina propria of loose connective tissue rich in blood vessels and lymphatic vessels, a coat of smooth muscle and an outer collagenous layer of adventitia
What is the function of the pancreas?
- Endocrine: small scattered islands of tissue which produce insulin and glucagon to regulate blood sugar
- Exocrine: produces pre-enzymes and digestive juices to break down proteins, lipids, DNA, RNA etc.
What is lipogenesis?
- Reductive process which is de novo synthesis of fatty acids occurring in liver, kidney, mammary glands, adipose tissue and the brain
- Takes place during excess energy intake
What is the formula for lipogenesis?
Dietary starch -> glucose -> pyruvate -> acetyl-coA -> malonyl-coA -> fatty acids -> triglycerides
Why is amino acid degradation needed?
There is no storage form for amino acids
What are the steps of amino acid degradation?
- Takes place in the liver
1) Absorption - by intestinal cells and released into blood
2) Protein turnover - tightly regulated, differing rates, damages proteins removed
What are the major-nitrogen containing excretory molecules
1) Urea
2) Uric acid
3) Creatinine
4) Ammonium ions
What is the formula for BMI?
Weight (kg) / square of height (m) = BMI
What are the reference ranges for BMI?
- Up to 25 = thin/normal
- 25.0 - 29.9 = overweight
- 30.0 - 39.9 = obese
- 40+ = morbidly obese
What are some satiation signals?
- Cholecystokinin (CCK)
- Peptide YY (PYY3-36)
- Glucagon-like peptide 1 (GLP1)
- Oxyntomodulin (OXM)
- Obestatin
What 2 hormones inform the brain to alter energy balance (↓less, ↑burn)?
- Leptin -> made and released from fat cells
- Insulin -> made and released from pancreatic beta cells
Why can leptin-therapy not be used in diet-induced obesity (DID)?
Severe leptin resistance due to:
1) Defective leptin transport into the brain
2) Altered signal transduction following leptin binding to its receptor
What is brown adipose tissue?
- Known as the ‘good fat’
- Main function is to turn food into body heat
What occurs in the orad region of the stomach?
- Fundus and proximal body
- No slow wave activity
- Relatively weak maintained tonic contractions of about 1 min, lasting for an hour
What occurs in the caudad region of the stomach?
- Distal body and antrum
- Phasic intermittent contractions
- Slow waves occur continuously (3 min-1) but only those reaching threshold elicit contraction
- Retropulsion mixes gastric contents, grinding chyme
How does gastric distension increase motility?
1) Stretch of smooth muscle
2) Simulation of intrinsic nerve plexuses
3) Increased vagus nerve activity and gastrin release
What factors control the strength of the antra wave and determine stomach emptying?
1) Rate of emptying proportional to volume of chyme in stomach
2) Consistency of chyme
3) Duodenum must be ready to recieve chyme
What secretions come from oxyntic (fundus and body) gastric mucosa?
- HCl
- Pepsinogen
- Intrinsic factor and gastroferrin
- Histamine
- Mucus
What secretions come from pyloric (distal stomach and antrum) gastric mucosa?
- Gastrin
- Somatostatin
- Mucus
What secretagogues induce hydrochloric acid secretion from the parietal cell?
1 - Acetylcholine (ACh)
2 - Gastrin
3 - Histamine
What inhibits hydrochloric acid secretion from parietal cells?
1 - Somatostatins
2 - Prostaglandins
What are the three phases of gastric acid secretion?
1) Cephalic - preparation by CNS and vagus nerves
2) Gastric - during, physical and chemical mechanisms
3) Intestinal phase - after, neuronal and hormonal mechanisms
What liquids are received by the small intestine?
- Chyme from the stomach
- Bile from the liver and gallbladder
- Pancreatic juice from the pancreas
How does mixing and propulsion of chyme occur in the small intestine?
1) Segmentation after meal - chopping moves chyme back and forth by alternating contraction and relaxation of circular muscle
2) Peristalsis when fasting - localised contractions and MMC
What is the migrating motor complex?
- Strong peristaltic contraction slowly passing length of the intestine every 90 - 120 mins
- Clears small intestine of debris, mucus and sloughed epithelial cells between meals
What hormones are secreted by the small intestine?
- Secretin from S cells
- Gastrin from G cells
- Cholecytokinin (CCK) from I cells
- Gastric inhibitory peptide (GIP) from K cells
- Glucagon like peptide 1 (GLP1) from L cells
- Motilin from M cells
- Ghrelin from Gr cells
What are the control mechanisms of the succus entericus?
- Distension/irritation (↑)
- Gastrin (↑)
- Cholecystokinin (CCK) (↑)
- Secretin (↑)
- Parasympathetic nerve activity (↑)
- Sympathetic nerve activity (↓)
What are the components of the success entericus of the small intestine?
- No digestive enzymes
- Mucus for protection/lubrication - by goblet cells
- Aqueous salts for enzymatic digestion - by crypts of Lieberkuhn
What are the pancreatic secretions?
1) Endocrine - insulin and glucagon to the blood
2) Exocrine - digestive enzymes (acinar cells) and aqueous NaHCO3- solution (duct cells) make up pancreatic juice, released to duodenum
How is pancreatic secretion controlled?
1) Cephalic - vagal stimulation of A cells
2) Gastric - vasovagal reflex -> parasympathetic stimulation of A and D cells
3) Intestinal - acid/fat/protein in duodenal lumen
What are the two types of digestive processes in the small intestine?
1) Luminal digestion - mediated by pancreatic enzymes secreted into the duodenum
2) Membrane digestion - mediated by enzymes situated at the brush border of epithelial cells
What is the sequence of carbohydrate digestion?
Polysaccharides (intraluminal hydrolysis) -> oligosaccharides (membrane digestion) -> monosaccharides (absorption)
Describe protein digestion in the stomach
- HCl begins to denature proteins
- Pepsin is an endopeptidase which cleaves proteins into peptides, has a preference larger neutral amino acids
What are the five pancreatic proteases?
1) Trypsin
2) Chymotrypsin
3) Elastase
4) Procarboxypeptidase A
5) Procarboxypeptidase B
Describe protein digestion in the duodenum
The five pancreatic proteases are secreted as proenzymes from the acinar cells of the exocrine pancreas and converted to active form in the duodenum; They function as either endopeptidases, or exopeptidases
What are the additional proteases which aid protein digestion other than pancreatic proteases?
1) Brush border peptidases
2) Cytoplasmic peptidases
How are amino acids absorbed?
- At brush border, 7 different mechanisms; 5 are secondary active transport; 2 Na+ independent
- At basolateral membrane, 5 different mechanisms; 3 mediate efflux of amino acids; 2 mediate influx
How are di-, tri- and tetra-peptides absorbed?
- Via H+-dependent mechanism at brush border (co-transport) pump is PepT1
- Further hydrolysed to amino acids within the enterocyte
- Na+-independent systems at the basolateral membrane (facilitated transport)
In what 3 ways does emulsification of ingested lipids occur?
1) Mouth - chewing
2) Stomach - gastric churning and squirting through narrow pylorus; content mixed with digestive enzymes from mouth and stomach
3) Small intestine - segmentation and peristalsis mix the luminal content with the pancreatic and biliary secretions
Describe the stabilisation of emulsion drops of ingested lipids
- Stabilised by the addition of a ‘coat’ of amphiphilic molecules that form a surface layer on the droplets
- Provide a vast increase in the surface area to volume ratio at which digestion by lipases and other esterases can accomplish digestion
What is the role of bile salts in the digestion of lipase?
- Emulsify large lipid droplets into small droplets
- Increase surface area for attack by pancreatic lipase
What is colipase and what is its role in lipid digestion?
- An amphipathic polypeptide secreted with lipase by the pancreas
- Binds to bile salts and lipase allowing lipase access to tri- and di-acylglycerols
How are digested lipids absorbed into small intestine cells?
- Lipid mixed micelles enter by passive diffusion/transport proteins
- Short and medium chain fatty acids enter through diffusion
- Long chain fatty acids are resynthesises into chylomicrons and carried in lymph to systemic circulation
- Cholesterol is transported by endocytosis in clatherin-coated pits
How is iron absorbed into small intestine cells?
- Reduced from Fe3+ to Fe2+ by accepting an e-
- Fe2+ absorbed across apical membrane by transport processes
- Conveyed to basolateral membrane via a molecular chaperone and trasported across by transporters
- Oxidised to Fe3+ and transported into tissues
- Imported across apical membrane; cytoplasmic metabolism to release Fe2+
How is vitamin B12 absorbed into small intestine cells?
Ingested in food bound to proteins -> stomach acid removes protein -> haptacorin binds to vitamin in stomach -> stomach parietal cells release intrinsic factor -> pancreatic proteases digest haptacorin in stomach -> vitamin B12 released -> bind to intrinsic factor in small intestine -> vitamin B12-intrinsic factor complex absorbed in terminal ileum by endocytosis
How are fat-soluble vitamins absorbed into small intestine cells?
- Incorporated into mixed micelles and passively transported into enterocytes or
- Incorporated into chylomicrons/VLDL’s and distributed by intestinal lymphatics
How are water-soluble vitamins absorbed into small intestine cells?
Transport processes in the apical membrane which may be either Na+-dependent, or Na+-independent
